1. Field of Invention
The present invention relates to a polarizing beam splitter typically used on optical pickups with which information is written to or read from optical recording media such as optical recording disks and magneto-optical recording disks.
2. Related Art
A polarizing beam splitter capable of diffraction at varying efficiency with the direction of polarization is used on various kinds of optical devices. Inexpensive models using an optically isotropic substrate have been proposed by the assignee in Unexamined Published Japanese Patent Application Nos. 230140/1997 and 66598/1999.
The polarizing beam splitters disclosed in these patents have essentially the same structure as the polarizing beam splitter of the invention and, as shown in FIG. 1 which is a schematic representation of the invention's model, has a glass substrate 11 as an optically isotropic substrate and an oriented polydiacetylene film 12a which is formed of a birefringent material to a predetermined thickness on the surface of the glass substrate. By patterned exposure to ultraviolet light, a periodic grating 12b consisting of alternating exposed and non-exposed areas of specified width is formed in the oriented polydiacetylene film 12a.
The present inventors have found a problem with these polarizing beam splitters, that is, after the formation of the periodic grating, thermal deterioration occurs to instabilize diffraction efficiencies such as the efficiency of first-order diffraction.
A further problem occurs when the polarizing beam splitter with a periodic grating is divided into discrete units by dicing the glass substrate in a pattern of squares. Since a jet of pressurized water is applied in the dicing step, the exposed areas of the oriented polydiacetylene film come off the glass substrate.